The D&RG class 70 locomotives (400-411) were delivered in black with significant line trimwork, as were most/all of the post-1880 D&RG locomotives. As the years went by, the line trim eventually disappeared. Boiler jackets were bare planished iron since paints of the period couldn't withstand high temperatures. Of course, as-built they also had diamond stacks, short smokeboxes, kerosene headlamps, fluted domes, and other typical 1880's features.
I suppose Knotts fixed up 340 in a manner that management thought looks nice with no particular regard for any specific place or time. It looks like something of a mish-mash of various historic periods sort of thrown together. Pretty much everything on it appeared on some locomotive, somewhere, at some time or another. You have 1880's-style brass boiler bands mixed with a turn of the century straight stack and comparatively modern electric lighting all on top of an excess of bright green paint which gives the whole thing a sort of Brazilian sort of look; some Brazilian railroads really liked Ivy Green. As Colorado history it, of course, fails miserably. However I don't think that's the point. As a locomotive in amusement park service I think it looks rather nice--bright and shiny without using circus colors (see some posts above), and the park already has one all-black locomotive anyway. I'm glad it's getting the attention.
You can go to Knotts' sister park Cedar Point if you want to see a sorry looking railroad. The colors are atrocious, straight out of the bad old days of the 1960's--fire engine red, puke green, etc, simultaneously garish and outdated, seldom even matching. When I was there this past summer the operating locomotive pulled around a ramshackle, home-built tender that appeared to be slapped together out of whatever spare parts and sheet metal the shop had laying around. They apparently couldn't even afford to paint their logo on the side. Another locomotive off to the side had a clearly home-built cab, with side panels roughly fit in place over what appeared to be former door openings. Several tracks were missing (relative to 20 years ago), the tops of the roofs of the cars had been modified to a plainer appearance, probably to ease maintenance, and so forth. Whether it's actually the case or not, the poor little railroad had the appearance of one suffering from lack of funding.